1. Field of the Invention
The present invention relates to an image reading apparatus adapted for use in a digital copier, a facsimile, an electronic file or the like, and more particularly to the positional adjustment of linear sensors performing image reading operation by relative movement with respect to the original image to be read.
2. Description of the Prior Art
The image sensor for reading image information for example in a digital copier, a facsimile or the like is often composed of CCD (charge-coupled device) line sensors. This is because the CCD line sensor is most suitable in the present technology for obtaining maximum the number of cells for converting the image information into electric signals. In order to improve the resolving power and to increase the signal transmission rate, there is proposed, for example in the U.S. Pat. No. 4,459,619, issued July 10, 1984 to T. Yoshida and assigned in common with the present application, an image reading apparatus in which plural CCD's are arranged along the main scanning direction.
The image reading apparatus utilizing the photoelectric converting elements as explained above does not require one-to-one projection as in the conventional copier in which the light reflected from the original is guided optically to a photosensitive member for exposure, because the CCD image sensor is composed of a semiconductor device containing a plurality of photoelectric converting cells on a small chip, and various advantages are derived from this fact. On the other hand, it is required to reduce the image of the original to the photosensitive area of the CCD image sensor, so that the mounting precision of the CCD image sensor has to be exactly controlled at least proportional to the reduction ratio. Also in the case that plural CCD image sensors are employed, it is required not only to position each image sensor in relation to the corresponding image position on the original but also to align the image sensors without superposition, skipping or bending so that the image reading with plural image sensors can be conducted as if it is done with a single image sensor.
For the purpose of adjusting the mounting positions of such CCD image sensors, there has been proposed a method of reading, prior to the start of original scanning, a black marker line which is provided outside the subsidiary scanning range (in FIG. 1B, outside the original 2 in the direction Y) and at the center in the main scanning direction (direction X in FIG. 1B) and mechanically adjusting the mounting position of the CCD image sensors in such a manner that the black level signal corresponding to said marker line occurs at a determined signal position, or taking out the effective signals alone from the output signals of the CCD image sensors, utilizing said marker line signal as the reference signal and storing said effective signal in a memory. More specifically, in case two CCD's are arranged along the main scanning direction and are moved in the subsidiary scanning direction to read the entire area of the original, in which the main scanning line is equally shared by two CCD's, a marker composed a black line of several millimeters wide is provided outside the subsidiary scanning area and at the center in the main scanning direction, and the CCD image sensors are arranged in such a manner that the photosensitive areas thereof partially overlap each other.
In such an arrangement, the positioning of the CCD image sensors is conducted in the following manner. At first it is necessary to adjust the longitudinal position, in the main scanning direction, of the CCD image sensors, for example by mechanical adjustment or optical projection, since the marker at the center in the main scanning direction assures the sensor position at said central position but not necessarily at the end positions. Then, prior to the original scanning, an area including the marker is illuminated by a light source to obtain serial electric signals from the first and second CCD's. Since the marker is positioned at the center in the main scanning direction, a signal corresponding to said marker appears at the rear end, or in the vicinity thereof, in the electric signals from one CCD and at the leading end, or in the vicinity thereof, in the electric signals from the other CCD. The width of said black marker line can be correlated in advance with the duration of the output signal from CCD. Consequently the positioning in the main scanning direction can be achieved by displacing the CCD's in said direction in such a manner that the portions of said black marker signal respective in the output signals of the CCD's are equal in duration.
The above-described conventional image reading apparatus in which the marker is provided at the center in the main scanning direction for positioning the CCD's has, however, been associated with the following drawbacks. Firstly, such a central marker is useless for confirming the parallel arrangement of CCD's in the main scanning direction, so that the positioning of the CCD's has to be made without relying on the marker and is quite time consuming. Secondly, in the case that three or more CCD's are arranged along the main scanning direction in order to further improve the resolving power, the positioning is even more tedious since the positional precision and the degree of parallel arrangement have to be improved proportional to the resolving power.